It is known to incorporate radar-absorbing material (RAM) into composite structures such as wind turbine blades. This is done to reduce the radar reflectivity of the blades so that they do not interfere with radar systems such as air traffic control systems or marine radar systems.
Many radar-absorbing materials are based upon the Salisbury Screen, which comprises three layers: a lossless dielectric layer sandwiched between a reflector layer or ‘ground plane’ and an impedance layer or ‘lossy screen’. The lossless dielectric is of a precise thickness equal to a quarter of the wavelength of the radar wave to be absorbed; the ground plane comprises a layer of highly reflective conductive material such as metal or carbon; and the lossy screen is generally a thin resistive layer.
Circuit analogue (CA) RAM technology has proven to be particularly effective for use in wind turbine blades. This is similar to the Salisbury Screen arrangement, but the impedance layer is replaced by a CA layer comprising an array of elements, such as monopoles, dipoles, loops, patches or other geometries. The elements form a pattern that repeats across the CA layer. The CA layer and the ground plane form a radar absorbing circuit in the composite structure.
It is known to embed a RAM impedance layer within a laminated composite structure such as a wind turbine blade. For example, FIG. 1a is a cross-section through an aerofoil part of a wind turbine blade 10, between a leading edge 11 and a trailing edge 12. The blade 10 is constructed from two aerodynamic shells, an upper shell 13 and a lower shell 14, which are joined together at join lines or seams that extend along the leading and trailing edges 11, 12 respectively. The shells 13, 14 are formed from a glass fibre cloth and resin composite. The shells 13, 14 are supported by a tubular structural spar 15 formed from glass fibre and carbon fibre.
FIG. 1b is an enlarged schematic view of the leading edge 11 of the blade 10, in which various layers comprising the shells 13, 14 can be seen. For ease of illustration the layers are shown separated, but in reality adjacent layers would abut. The shells 13, 14 each comprise a skin 16 of composite construction and formed from one or more layers of glass-fibre fabric within a hardened resin matrix. A CA layer 17 is deposited on an outer surface of the skin 16. A gel coat 18 covers the CA layer 17. A ground plane 19 comprising a thin layer of carbon veil, is adhered to an inner surface of the skin 16 such that it is in spaced apart relation from the CA layer 17. The CA layer 17 and the ground plane 19 act together to form a radar absorbing circuit.
When constructing the blade 10, each of the shells 13, 14 are moulded separately and then joined together. To make a shell 13 or 14, the various glass-fibre fabric layers comprising the skin 16 are laid up in a gel-coated mould. The layers may be infused with resin in the mould, or the layers may be pre-impregnated with resin (prepreg). The resin is subsequently hardened in a curing process. The CA layer 17 is pre-printed or otherwise deposited on a surface of one of the glass-fibre fabric layers prior to layup so that the CA layer 17 becomes embedded within the resulting composite structure. A prepreg material suitable for use in the above-described moulding process to provide an embedded CA layer is described in WO2010/122350. The prepreg material comprises an impedance layer deposited onto a resin-impregnated glass-fibre layer.
Whilst an embedded CA layer 17 works well in many cases, it has been found that this arrangement works less well at joins in a composite structure, for example at the join 20 between the upper and lower shells 13, 14 at the leading edge 11 of a wind turbine blade 10. This is because the repeating pattern of the CA elements is inevitably disrupted at the join, which can result in reduced RAM performance. The present invention aims to overcome this problem.